The present invention relates to hydrostatic piston machines used in power transmission and transaxle product and is concerned with an improved pump and motor configuration. Although in most instances, single motor hydrostatic transmissions are the normal, some applications do exist where two hydraulic motors are required.
Hydrostatic transmissions act as means for converting rotary mechanical motion, typically provided by an internal combustion engine, to fluid motion by means of a shaft driven hydraulic pump and where the pump is fluidly connected to one or more hydraulic motors. The volume and direction of the fluid delivery to the motor or motors being governed by the displacement setting of the pump, and the motor is the mechanism for converting the fluid motion back to mechanical rotary motion. By regulating both the amount and direction of fluid motion between the pump and motor, usually achieved through the use of a variable-displacement pump, the output speed and rotational direction of the motor can be precisely controlled at a set input rotational speed of the pump. This device is useful in driving vehicles such as lawn and garden tractors or any apparatus requiring precise speed control. In many such applications, the hydrostatic transmission is ideally integrated as part of an overall transaxle driving apparatus which further incorporates speed reducing means and, in the case of the single motor arrangement, a differentialled axle as shown in U.S. Pat. No. 4,979,583 entitled Variable Speed Transaxle.
Hydrostatic transmissions of the type which are fully integrated in a transaxle housing structure containing speed reduction gearing and mechanical differential are fast becoming the preferred driving apparatus for lawn tractors and the like, and are currently either of the type where the hydrostatic transmission utilises an axial piston swash-plate configuration for both the pump and motor or a radial piston configuration for both the pump and motor.
In the axial piston variety of integrated hydrostatic transaxles, the pump and motor are fluidly connected together by means of a center-section which generally is a xe2x80x9cLxe2x80x9d section configuration containing both fluid passages and valves as well as valve-plate surfaces either formed integral to it on two exterior surface arranged perpendicular to one another on the center-section or as separate components mounted and fixed to such surfaces on the center-section. An example of such axial piston hydrostatic transaxles is shown in the Okada et al. U.S. Pat. No. 4,914,907 and units built to this design, although proven to be both durable and performance worthy, are still significantly more expensive to produce compared to the less sophisticated mechanical gear-shift transmission/transaxle.
At the present time, the need to include such a center-section, which due to its complexity, is the single most expensive item required in the axial piston pump and motor configuration, is a serious obstacle hindering further price reduction. A further problem exists because center-sections are constructed from die-cast or cast aluminium material and porosity in the material surrounding the high-pressure internal fluid passages can, if present to any significant degree, result in a loss of operating efficiency due to the leakage of high-pressure fluid. A still further problem presents itself when the aluminium material is used as the operating surfaces on which the cylinder-barrels revolve. As aluminium is not so durable as compared to steel or iron, the surfaces can wear out prematurely, again resulting in a loss of operating efficiency due to increased leakage of high-pressure fluid.
With respect to the alternative type of hydrostatic unit currently sold in the market and which employs a radial piston pump and radial piston motor configuration, the fluid connection is, by contrast, achieved simply and more economically by using a pintle-valve. Such a pintle-valve is simple to produce and compares extremely favourably in terms of manufacturing cost with their center-section counterpart used in the axial piston design. Pintle-valves are constructed in steel and are heat treatable to provide a very strong and hard wearing operating Subsurfaces against which the cylinder-barrels revolve. Even so, because the standard orientation in the drive-line installation for almost all lawn and garden vehicles, irrespective whether the hydrostatic transaxle uses radial pistons or axial pistons, requires a 90 degree shift in the rotational axis between the input shaft and the axle output shafts, the radial piston type of hydrostatic unit requires additional components in the form of bevel gearing. Such bevel gearing can be positioned behind the hydraulic radial piston motor as is shown in Thoma et al. U.S. Pat. No. 4,979,583 or alternatively in front of the hydraulic radial piston pump as is shown in von Kaler et al. U.S. Pat. No. 5,078,659 However, in either arrangement, the need to include such bevel gearing increases the selling price of the product and may unless the gearing correctly mesh, result in higher than desirable generated gear noise during operation.
The present invention is directed at overcoming the above described disadvantages by way of using the best features of the radial piston pump configuration and the best features of axial piston motor configuration whereby the center-section of the type shown in Okada et al is redundant and the bevel gears shown in Thoma et al or von Kaler et al are redundant. A desirable feature of the radial piston pump is that high speed rotation of the piston carrying cylinder-barrel produces a centrifugal effect on the pistons propelling them radially outwards from their cylinders to improve the draw of fluid into the cylinder. A desirable feature of the axial piston motor design having stick or cylindrical pistons is that the stroke to piston diameter ratio is always greater than the radial piston motor. As a result, based on the same diameter of piston and same number of cylinders, the axial piston motor provides an automatic hydraulic displacement reduction whenever fluidly coupled to a radial piston pump, and this has the great advantage that less speed reduction is needed between the motor and the output power transmission shaft or shafts.
From one aspect the invention consists of a hydrostatic transmission located within a chamber defined by a surrounding housing structure and comprising a hydraulic pump and at least one hydraulic motor fluidly coupled together; an input drive-shaft rotatably supported in said housing and drivingly connected to said hydraulic pump, said pump having a first cylinder-barrel containing an array of radially arranged cylinders and a radial piston disposed in each of said radial cylinders and where the stroking axes of said radial pistons all lie along a common plane normal to the rotational axis of said first cylinder-barrel; said at least one motor having a second cylinder-barrel containing an array of axially arranged cylinders set equally spaced apart about a circle described by the stroking axes of axial pistons disposed in said axial cylinders to lie radially within the radial outer dimension of said second cylinder-barrel such that the stroking axes of said axial pistons lie parallel to the rotational axis of said second cylinder-barrel.
Although the embodiment used to illustrate this invention is a transaxle unit useful for a vehicle application where a differential action is required between the drive wheels, the invention is also applicable to transaxles where no differentialled action is required between the respective output axle shafts, either by omitting the differential or by adding an extra hydraulic motor in the transaxle in place of the differential. Furthermore, the invention is useful for a self-contained power transmission unit having a single or double output power transmission shaft that may or may not include speed reducing gearing within the same housing package.
The cylinder-barrel of the radial piston pump is driven by an input power transmission drive-shaft, either through some preliminary gearing or, preferably directly, and where the cylinder-barrel is supported for rotation on a cylindrical surface on a fluid coupling valve. Radial pistons are disposed within an array of radial cylinders provided in the cylinder-barrel and protrude from their respective cylinders to be operatively connected to a surrounding annular track-ring, and where the track-ring is adjustable is its position relative to the radial position of the cylinder-barrel such that its eccentricity can be changed enabling the delivered fluid medium to be varied in both quantity and direction to the hydraulic motor. The cylinder-barrel of the axial piston motor includes a plurality of axial cylinders in which axially sliding pistons are contained. Springs between each axial piston ensure that the cylinder-barrel is biased towards a flat surface on a fluid coupling valve and where the cylinder-barrel is connected to a drive shaft for the onward transmission of mechanical power. The axial pistons protrude from their respective axial cylinders and are operatively connected to an angle thrust plate, also called a fixed-angle swash-plate. Fluid delivered by the pump radial pistons is arranged to pass through internal passages in the fluid coupling valve to enter each of the axial cylinder chambers in turn that are provided in the cylinder-barrel of the hydraulic motor.
With the single pump/single motor configuration, the fluid coupling valve comprises a cylindrical element and a block element; and where the cylindrical element supports the cylinder-barrel of the radial piston pump and where the block element has a flat surface provided on which the cylinder-barrel of the hydraulic motor contacts. In the region where the cylindrical and block elements contact their respective cylinder-barrels, a pair or arcuate-slots are provided to allow fluid within the fluid coupling valve to enter through cylinder-ports the cylinders provided in each of the cylinder-barrels. The cylindrical element may be extended axially to extend beyond and below the pump cylinder-barrel to interface with a concave part-cylindrical surface formed on the block element. When two motors are required, the block element is provided with a second flat support surface such that each cylinder-barrel of the two hydraulic motors has its own flat support surface to contact. Alternatively, there may be two block elements used in the two motor hydrostatic transmission, and in this case, the downwardly extending portion of the cylindrical element interfaces on one side with a concave part-cylindrical surface formed on one of the block elements as well as with a concave part-cylindrical surface formed on the other block element on the opposite side.
It is an object of the invention to provide a improved hydrostatic transmission or transaxle that is both ecomonic and compact, especially in terms of height, where the relatively flat pancake shaped radial piston pump is superimposed above the more cylindrically shaped axial piston motor or motors.
A object of the invention to exploit the nature advantages inherent in the respective radial piston pump and axial piston motor configurations in order to reduce the amount of final drive train gearing necessary to accomplish a multiplication in torque to the drive wheels of the vehicle. In the example set forth, the usual stroke to diameter ratio of the radial piston ball pump is 30% whereas the usual stroke to diameter ration of the axial piston stick motor is 70% thereby providing a greater than 2:1 hydraulic reduction assuming same diameter and number of cylinders in the pump and motor.
It is a further object of the invention to provide a simple but effective interface between the cylindrical element and the block element of the fluid coupling valve in a manner whereby the 90 degree shift in the axis of rotation between the radial piston hydraulic pump and the axial piston hydraulic motor or motors can be effected without the complexity of the prior art where either, bevel gears are required, or expensive and complex centre-sections are needed which suffer on occasion due to porosity or wear problems associated with the die-cast aluminium material.
It is a still further object of the invention to provide a more durable transmission or transaxle product whereby the critical operating surfaces on which the cylinder-barrels of both hydraulic pump and hydraulic motor revolve is constructed of a hard wearing surface of steel or iron material. In terms of the axial piston hydraulic motor, this invention allows the use of a sintered powder-metal iron valve-plate product in place of the aluminium material used in the prior art types incorporating an aluminium center-section.